DiagAccess and dtt2bunch don't support references
References can be stored in a file in an incomplete fashion, i.e. a transfer function can be saved as a reference without the corresponding coherence record for the same channels.
DTTXML requires a coherence array for each TF loaded, so fails when loading TFs as references.
DTT should probably save all associated calculations with a TF reference, but this is a DTTXML idiosyncrasy, not a necessity, and could be done away with. At the very least, it should throw a clearer exception.
@jameson.rollins has a branch, ref-checks
, that addresses the problem.